FDNY dispatch protocol/policies are very hard to master a full understanding of without firsthand knowledge. Things often change and seemingly similar events(from an outside perspective) may warrant different responses. I think in this instance because they were called for relief the 4th wasn't transmitted. I've seen situations where special calling 2 engines for relief required next alarm transmission. I also remember the 6th alarm/10-87 on W. 29 in 2016 where it appeared units were gaining good ground and even went PWH, I left the scene walked around midtown for a little while and the fire had taken off big time, heavy volume of fire. 4 engine 2 ladder rescue & squad all in emergency mode were all special called and yet PWH was not rescinded and it never went to 7th alarm.